1. Field of the Invention
The present invention relates to a constant voltage circuit which supplies power to a load in an electronic device or an integrated circuit, and more specifically to an overcurrent protection circuit which prevents an overcurrent of the constant voltage circuit.
2. Background Art
A constant voltage circuit is required in order to obtain a desirable power supply voltage in an electronic device or an integrated circuit. The constant voltage circuit has a capability of outputting constant voltage and supplying power to a load. An overcurrent protection circuit is required in order to avoid a problem of a heat generation or the like caused by excessive power supplied when an output load of the constant voltage circuit flows to a large current or is short-circuited, and in order to obtain overcurrent protection characteristics with a good accuracy, various overcurrent protection circuits are proposed (for example, Patent Document 1).
An example of a circuit diagram of the constant voltage circuit including the overcurrent protection circuit of the related art is illustrated in FIG. 8.
An error amplifier 102 of the constant voltage circuit of the related art compares a reference voltage output from a reference voltage source 101 with a feedback voltage which is generated by dividing a voltage of an output terminal Vout using a voltage division circuit 104, and outputs a voltage which controls an output transistor 105 such that an output voltage is constant, and thereby the constant voltage circuit operates as a constant voltage circuit.
The overcurrent protection circuit 103 of the related art includes an output current sense transistor 106 which senses an output current and controls a PMOS transistor 107 based on a sense current output from the output current sense transistor 106, and operates such that the output current of the output transistor 105 does not become a current equal to or more than a predetermined limit current. The overcurrent protection circuit 103 is a drooping type overcurrent protection circuit.
In addition, the overcurrent protection circuit of the related art includes an output voltage detection circuit that is configured by an output current sense transistor 115 which supplies the sense current, an NMOS transistor 116 through which the sense current flows, an NMOS transistor 117 which configures a current mirror together with an NMOS transistor 116, a PMOS level shifter 118 through which a current proportional to the sense current flows, a PMOS level shifter 119 whose gate receives a drain voltage of the PMOS level shifter 118. The output voltage detection circuit controls such that the drain voltage of the output current sense transistor 115 is equal to the voltage of the output terminal Vout by the PMOS level shifter 119. Further, the drain voltage of the PMOS level shifter 118 is input to the gate of the PMOS level shifter 120, and thereby the control is performed in such a manner that the drain voltage of the output current sense transistor 106 becomes equal to the voltage of the output terminal Vout. By this configuration, a source-drain voltage of the output transistor 105 becomes equal to a source-drain voltage of the output current sense transistor 106, and thereby it is possible to obtain overcurrent protection characteristics with a good accuracy, although a voltage difference between an input terminal Vin and the output terminal Vout is small.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-029856